Joint Actuator Design
4.3 Control Componentry (Rotational Actuator)
4.3.1 Non-Self-Locking Motor
Size and power output was a large aspect of motor selection. Polulu has a range of geared motors of varying properties. The 99:1, medium-power Gearmotor was selected to provide a non-self-locking, low power option for the rotational actuator.
It was found that, when unpowered, this motor required just enough torque to hinder manipulator movement under its own weight, but provided free movement when exerting an external torque. In contrast, the manipulator becomes self-locking when powered. Final rotational speed was required to be of a magnitude that allows the actuator to traverse its full range in no more than 7 seconds. Output torque needed to exceed 6.5 Nm in order to actuate a link with two rotational actuators, a 90-deg bend and gripper attached.
The non-self-locking nature exhibited by this motor is useful to the final system, allowing the user to manually move the manipulator to a desired position, so that an arm location may be captured in software for later use.
Gearing:
Motor pinion no. teeth 15 Internal gear no. teeth 120
12 × 120
15 = 96 = 9.4
Rotational Velocity:
76 × 15
120 = 9.8 = 6.2 /
Figure 4 - 10. Polulu 99:1 metal gearmotor MP 12V.
Weight 91g Gear ratio 98.78:1 Free-run speed 76 rpm Free-run current 12V 200 mA Stall current 12V 2100 mA Stall torque 12 kg-cm
4.3.2 Self-Locking Motor
Although non-self-locking is useful in adding to the usability of the final system, none of the available medium-power motors provide enough torque for larger configurations, without severely impacting rotational velocity. The motor, placed at the base of the articulated configuration, would need a calculated maximum torque of 15 Nm to move the manipulator. As a consequence, self-locking, high-power motors were investigated as a means of arm actuation.
The 25 mm diameter, high-power geared-motor package does not allow for any rotation when unpowered. This proved to be very useful in terms of power usage, as the control signal may be cut from these motors once they are at their desired position. In contrast, the medium-power motors require a constant signal to stay self-locking.
Gearing: 21 × 120 15 = 168 = 16.5 Rotational Velocity: 100 × 15 120 = 13 = 4.8 /
Figure 4 - 11. Polulu 99:1 metal gearmotor HP 12V.
Weight 91g Gear ratio 98.78:1 Free-run speed 100 rpm Free-run current 12V 300 mA Stall current 12V 5600 mA Stall torque 21 kg-cm
4.3.3 Motor Driver
The Polulu G2 high-power motor driver was found to be ideal for the proposed system in terms of size, power output and signal control. This small, discrete MOSFET H-bridge package is capable of delivering a continuous 13 A and includes basic current-sensing and limiting functionality. The current sense output voltage is stated to be 40 mV/A, with an offset of 50 mV.
Unfortunately the Arduino Micro analogue pins have a read range that cuts 5V into 1024 segments. This results in a maximum voltage read sensitivity of 49 mV, which is too large to provide an accurate motor current calculation. A precise current reading may have resulted in the ability to calculate rough motor output torque estimations, which may have been very useful. Nevertheless, this sensitivity is still of the magnitude to inform the microcontroller if current usage spikes excessively.
Figure 4 - 12. Polulu G2 High-Power motor driver 24v13.
A current limiting reference resistor was added to the driver, corresponding to power of the type of motor it was to drive, as per Figure 4 - 13, offering extra over-current protection.
Figure 4 - 13. Polulu G2 24v13 driver current limit reference.
Weight 3.3g
Min operating voltage 6.5 V Max operating voltage 40 V Continuous output current 13 A
Current Sense 0.04 V/A Maximum PWM frequency 100 kHz
Logic operating voltage 5V
4.3.4 Encoder
Since the rotational actuator design calls for an encoder mounted directly to the actuated shaft, it was desirable for the resolution to be of a higher magnitude. As such, an absolute 12-bit magnetic encoder was used with 4,096 positions per revolution, translating into a resolution of ~0.09 degrees. With an offset link length of 259 mm, the resulting calculated maximum end accuracy resolution per link is 0.4 mm.
US Digital provides two, electronically identical versions of the encoder, the MAE3 shaft encoder and the MAE3 encoder kit (fig. 4 - 14). The encoder kit was favoured as it is documented to tolerate shaft axial play of up to ± 0.635 mm.
Figure 4 - 14. MAE3 encoder pictures.
Resolution 4096 positions per revolution
Operating voltage 5V
Typical current usage 16 mA
Sampling rate 250 Hz
Table 4 - 7. MAE3 encoder specifications.
Encoder position is determined by its output duty cycle ‘ON’ / ‘OFF’ ratio (fig. 4 - 15). 12-bit duty cycle position calculation:
= × 4098
+ − 1
≤ 4094, =
= 4096, = 4095
4.3.5 Micro-Controller
The Arduino Micro is a powerful microcontroller, consisting of some basic control componentry to better serve its integrated ATmega32U4 chip. This chip is available by itself, however it was chosen to go for the entire board, as this may be easily replaced in the event of catastrophic failure. The on-board voltage regulator also increased the ease- of-use for this package.
Figure 4 - 16. Microcontroller pinouts.
It was decided to go for Arduino as there is significant documentation available for both the hardware and the proprietary software accompanying the system. The number and type of I/O pins provided by the Micro were also found to be adequate.
Microcontroller ATmega32U4
Operating voltage 5V
Input voltage range 7-12V
Digital I/O pins 20
PWM channels 7
Analogue input channels 12
Clock speed 16 MHz
Weight 13g